Time-delay and integrate (TDI) image scanning is a common method for imaging wide areas or for improving the signal-to-noise ratio (SNR) of a dim scene. TDI dates back to the age of film. It was developed for aerial reconnaissance to solve the problem of image smear and improve the SNR. TDI has also been used for letter and film scanning, aerial reconnaissance, digital radiography, and satellite mapping.
In film-based TDI imaging, film rolls by an open camera shutter at the same relative velocity as the target image to produce a long strip of film of an apparently static scene. Today, most TDI cameras use charge-coupled device (CCD) image sensors instead of film. Instead of rolling film, the registers of the CCD are clocked in synchronization with the object velocity. Put differently, the charge is coupled from row to row in the CCD at a rate that matches the relative velocity of the target image. The charge is integrated as it traverses the CCD to produce a TDI image that may be orders of magnitude brighter than an equivalent static image.
TDI offers several advantages over conventional imaging: (1) it can provide higher SNR; (2) it eliminates the need for a mechanical shutter; (3) it enables long exposure times without introducing smear; and (4) it allows capture of tremendous amounts of high-resolution imagery from a relatively small camera when compared to an equivalent framing camera. For instance, a TDI CCD with 64 rows can collect 64 times as much signal, resulting in an 8-fold increase in SNR.
But CCD-based TDI cameras suffer from drawbacks as well. In particular, a typical CCD has a small charge well depth. This small charge well depth limits the total amount of charge that the CCD can integrate as the charge is transferred across the CCD during the TDI period. As a result, the charge well depth limits the dynamic range of the TDI CCD.
TDI systems can benefit from an extended dynamic range. For example, a TDI system having an extended dynamic range could employ slower scan rates and longer integration times, resulting in greater achievable sensitivity. Alternatively or in addition, digital pixel-based TDI systems having an extended dynamic range could be built that use fewer counter bits to achieve the same dynamic range as regular, non-extended dynamic range systems. This would offer an advantage of using smaller pixels in the extended dynamic range system so that greater resolution imagery could be obtained.